Sintered Nd-base magnets have a growing range of application due to their excellent magnetic properties. Also in the field of rotating machines including motors and power generators, permanent magnetic rotating machines utilizing sintered Nd-base magnets were developed to meet the recent demand for size, profile and weight reductions, performance enhancement and energy saving.
From the structural aspect, the permanent magnetic rotating machines are classified into radial gap and axial gap structures. The radial gap-type rotating machine comprises a rotor comprising a plurality of circumferentially arranged permanent magnets each having radially directed magnetic poles, and a stator opposed to the permanent magnets. In general, the stator includes an iron core having a plurality of teeth on its surface facing the rotor, with coils wound on the teeth. The use of the iron core allows magnetic fluxes from the poles of the rotor to efficiently interlink the coils. This produces an increased torque in the case of motors or a high voltage in the case of power generators. Since coils are wound in the stator, coil ends axially protrude from the magnetic circuit section of the motor whereby the motor is axially prolonged. When a motor having a reduced axial size is necessary, the radial gap-type rotating machine is inconvenient to use, and the axial gap-type rotating machine is rather advantageous.
Permanent magnets in axial gap-type rotating machines are exposed to high temperature due to the heat generated by windings and cores and have a likelihood of demagnetization by the demagnetizing field from the windings. There thus exists a demand for sintered Nd base magnets in which the coercive force which is an index of heat resistance and demagnetization resistance is above a certain level and the remanence (or residual magnetic flux density) which is an index of the magnitude of magnetic force is as high as possible.
Further, sintered Nd base magnets are conductors having an electric resistance of 100 to 200 μΩ-cm. As the rotor rotates, the magnet undergoes a variation of magnetic flux density, by which eddy currents flow. Effective means for reducing eddy currents is to divide a magnet body to interrupt the eddy current path. While division of a magnet body into smaller pieces leads to a more reduction of eddy current loss, it becomes necessary to take into account such problems as an increase of manufacturing cost and a lowering of output due to a reduction of magnet volume by increased interstices.
The eddy current path runs in a plane perpendicular to the magnetization direction of a magnet, with a higher current density prevailing in an outer peripheral portion. The current density is also higher at a side closer to the stator. That is, the amount of heat generated by eddy currents is greater near the magnet surface, so that the magnet surface region assumes a higher temperature and becomes prone to demagnetization. To suppress demagnetization by eddy currents, a sintered Nd base magnet in which the coercive force which is an index of demagnetization resistance is higher in the magnet surface region than in the magnet interior is required.
Several measures are known to improve the coercive force.
An increase in the remanence of sintered Nd base magnet is achieved by increases in the volume fraction of Nd2Fe14B compound and the degree of crystal orientation, and various improvements in process have been made therefor. As to an increase in coercive force, there are known various approaches including formation of crystal grains of finer size, use of an alloy composition having an increased Nd content, and addition of an effective element. Of these, the currently most common approach is the use of an alloy composition having Dy or Tb substituted for part of Nd. By substituting such elements for Nd of Nd2Fe14B compound, the compound is increased in anisotropic magnetic field as well as coercive force. On the other hand, the substitution of Dy or Tb decreases the saturation magnetic polarization of the compound. Accordingly, the attempt to increase the coercive force by the above approach fails to avoid a lowering of remanence.
In sintered Nd base magnets, the coercive force is given by the magnitude of an external magnetic field created by nuclei of reverse magnetic domains at grain boundaries. Creation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping creation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force (See K. D. Durst and H. Kronmuller, “COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75).
By concentrating trace Dy or Tb only in proximity to the grain boundaries to increase the anisotropic magnetic field only in proximity to the boundaries, the coercive force can be increased while suppressing any decline of remanence (see JP-B 5-31807). Subsequently, the inventors established a production method comprising separately preparing a Nd2Fe14B compound composition alloy and a Dy or Tb-rich alloy, mixing them and sintering the mixture (see JP-A 5-21218). In this method, the Dy or Tb-rich alloy becomes a liquid phase during the sintering and is distributed so as to surround the Nd2Fe14B compound. As a consequence, substitution of Dy or Tb for Nd occurs only in proximity to grain boundaries in the compound, so that the coercive force can be effectively increased while suppressing any decline of remanence.
However, since the two types of alloy fine powders in the mixed state are sintered at a temperature as high as 1,000 to 1,100° C., the above-described method has a likelihood that Dy or Tb diffuses not only to the boundaries, but also into the interior of Nd2Fe14B grains. An observation of the structure of an actually produced magnet shows that Dy or Tb has diffused to a depth of about 1 to 2 μm from the boundary in a grain boundary surface layer, the diffused area reaching 60% or more, calculated as volume fraction. As the distance of diffusion into grains becomes longer, the concentration of Dy or Tb near the boundaries becomes lower. An effective measure for positively suppressing the excessive diffusion into grains is by lowering the sintering temperature. However, this measure cannot be practically acceptable because it compromises densification by sintering. An alternative method of sintering at lower temperatures while applying stresses by means of a hot press or the like enables densification, but poses the problem of extremely reduced productivity.
On the other hand, it is reported that coercive force can be increased by machining a sintered magnet to a small size, depositing Dy or Tb on the magnet surface by sputtering, and heat treating the magnet at a temperature lower than the sintering temperature, thereby causing Dy or Tb to diffuse only to grain boundaries (see K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida, H. Kawasaki, T. Suzuki, M. Ito and T. Horikawa, “Grain Boundary Tailoring of Sintered Nd—Fe—B Magnets and Their Magnetic Properties,” Proceedings of the 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202). These methods allow for more effective concentration of Dy or Tb at grain boundaries and succeed in increasing the coercive force without a substantial loss of remanence. As the magnet becomes larger in specific surface area, that is, the magnet body becomes smaller, the amount of Dy or Tb fed becomes larger, indicating that this method is applicable to only compact or thin magnets. However, there is still left the problem of poor productivity associated with the deposition of metal coating by sputtering or the like.
WO 2006/043348 discloses means for efficiently improving coercive force which has solved the foregoing problems and lends itself to mass-scale production. When a sintered R1—Fe—B magnet body, typically sintered Nd base magnet body is heated in the presence of a powder on its surface, the powder comprising one or more of R2 oxides, R3 fluorides, and R4 oxyfluorides wherein each of R1 to R4 is one or more elements selected from among rare earth elements inclusive of Y and Sc, R2, R3 or R4 contained in the powder is absorbed in the magnet body, whereby coercive force is increased while significantly suppressing a decline of remanence. Particularly when R3 fluoride or R4 oxyfluoride is used, R3 or R4 is efficiently absorbed in the magnet body along with fluorine, resulting in a sintered magnet having a high remanence and a high coercive force.